Make module refs in description hyperlinks

I kept getting annoyed by not being able to click these, having to hunt them down in the module listing manually.

Downsides to this solution:

* The links are for the latest built version on Hackage, even if you're browsing an older version
* If you install with `--enable-documentation` these links will go to Hackage and not your local offline documentation
* It needs a recent Haddock to render correctly, which isn't a problem on Hackage but could be with locally build documentation

The upside, of course, is that you now *can* click the module references in the description on the Hackage page for pipes.

Pull at your discretion!